You're digging into a pairing with surprisingly deep fandom roots, aren't you? The Code Geass fandom has this weird history where the 'rivalry' fics dominated for years, but the true crossovers—where the 'Lelouch of the Rebellion' and 'Suzaku Kururugi' from entirely different worlds meet—are a specialized taste. Most of the good stuff isn't neatly tagged. You have to hunt on sites where writers experiment with world-building mechanics more than just romance.
AO3 is the obvious starting point, but filtering is key. Use the 'Alternate Universe - Crossover' tag paired with both character names, then sort by kudos. The real gems often involve fusion universes, like a 'Zero' appearing in Suzaku's 'Knightmare Frame' timeline from another series. That said, the absolute best one I've read, 'The Unforgotten Equation,' was on a private, now-defunct LiveJournal community dedicated to mecha anime crossovers. The scene where Lelouch's Geass fails against a Suzaku trained by, say, the Gundam Wing pilots... that specific clash of ideologies and power systems is where this pairing shines outside their original script.
Ultimately, it's less about a single platform and more about following prolific writers who enjoy deconstructing their moral codes in new settings. They tend to migrate, but their backlogs are gold.